1. Field of the Invention
The present invention relates to an automatic exposure control device to automatically control the exposure value of a camera, and more particularly, to an automatic exposure control device to convert the analogue value of an object, to digital value for obtaining appropriate exposure amount based on said digital value.
2. Description of the Prior Art
Heretofore there have been a number of patent applications related to a method to automatically control exposure of camera. Many of them can be classified into the following three kinds. That is:
(1) A method to control aperture value of a taking lens as a result of computation of brightness of an object to be photographed, film sensitivity and shutter speed;
(2) A method to control shutter speed as a result of computation of brightness of an object to be photographed, film sensitivity and aperture value of a taking lens, and
(3) A method to control predetermined combination of the aperture value of a taking lens and shutter time as a result of computation of object brightness and film sensitivity.
While each of these methods has special features and has been placed in practical use, it has shortcomings, also. That is, in the method of (1), since a photographer has the freedom to select freely shutter speed beforehand, he can relate the movement of an object to the shutter speed by selecting fast shutter speed for an object with fast movement. As far as the depth of field is concerned, however, the result obtained by the aperture value of a photographing lens determined automatically within the device according to the predetermined shutter time must be followed. When a stationary object is to be photographed, the depth of field is the most important element for composition of picture, therefore the method (1) is not satisfactory in this respect.
In the method of (2), the situation is totally reverse to the case of (1). That is, since shutter time is not left to the choice of a photographer, longer exposure time is given to an object with fast motion resulting in a picture with blur.
In the method of (3) since the combination of diaphragm aperture value and shutter speed will be determined simply for a given brightness of the field of photographing, no considerations are given for providing appropriate exposure. As a counter measure for such situation, such exposure control device has been proposed that the above mentioned shortcomings of the conventional device is eliminated, wherein a change over means is provided on an exposure control mechanism to enable the photographer to make so-called diaphragm aperture preference photographing, that is, as film sensitivity and diaphragm aperture value are set beforehand corresponding to the state of an object, exposure time can be controlled corresponding to the brightness of an object also to enable the photographer to make so-called shutter time preference photographing, that is, as film sensitivity and exposure time are set, the diaphragm aperture value can be controlled corresponding to the brightness of an object.
However, such conventional device which enables said two kinds of preference photographing to be done has its exposure control mechanism mechanically composed, therefore it has shortcomings that it becomes very complicated and the device itself become large in size.
Also a method to exercise said exposure control electrically has been proposed, but since majority of its control circuit is analogical electrical circuit, functional properties vary depending on the temperature of each circuit thus exposure control with high degree of accuracy could not have been done. An element with high degree of accuracy should be used for each position required to obtained high degree of accuracy in exposure control, but said element will cost high pushing the cost up thus it has a shortcoming of being uneconomical.
Further, to apply the above mentioned exposure control system having double preference function to a single lends reflex camera with TTL photo-sensing system which is now widely used for photographing of high level is considered to be very difficult because of its structure. Thus such camera with double preference function has not been placed in practical use although the need and advantages for the same have been recognized.